The coherently coupled nonlinear Schrdinger （NLS） equation of the propagation of a light pulse in a fiber has been utilized. The propagation characteristic of linearly polarized light in a nonlinear birefringent dispersiondecreasing fiber has been studied. The results show that when a linearly polarized wave propagates in single mode fiber. It may change polarization state due to interaction among linear birefringence

nonlinear effect and chromatic dispersion. It will produce cross phase modulation and lead to phasematched parametric fourphoton mixing .The propagation can be produced not only in the normal dispersion regime but also in anomalous dispersion regime. Furthermore

it will change with propagation distance.